The present invention relates to oil-based corrosion inhibitor additives for combination with lubricating or hydraulic oils. When added to the oils at a concentration of between about 2–5% by weight, the combination enhances protection of ferrous and nonferrous metals. The additives effectively prevent corrosion of engines during intermittent operation and storage, and decrease high-temperature oxidation during use. This new inhibitor system is low in toxicity, is thermally stable and provides long-term corrosion protection. It further provides protection in the presence of chlorides and does not affect oil pour point and viscosity at low temperatures.